1. Field of the Invention
The present invention relates to a power supply for a vehicle, and more particularly, to a technology for composing a power supply of two systems with different voltages.
2. Description of the Related Art
Conventionally, it has been tried to reduce current flowing through wiring in a vehicle and to reduce the wiring in weight by increasing the power supply voltage of a power supply for a vehicle. However, for instance, loads such as lamp are difficult for application of a high voltage, and these loads are supplied with a low voltage obtained by converting the high voltage through a voltage converter. Consequently, this kind of power supply for a vehicle is provided with a power supply of two systems such as high voltage system and low voltage system.
As an example of such a power supply provided with a power supply of two systems, Japanese Patent Application Laid-Open (JP-A) No. 1-185197 discloses xe2x80x9ca Power supply apparatus for a vehiclexe2x80x9d. This power supply for a vehicle is provided with a switch element between a high voltage system and a low voltage system, and generates a voltage in rectangular pulse-wave with a predetermined duty by switching this switch element in ON or OFF. Then, a power with this rectangular pulse-wave voltage is supplied to a load as a power supply of the low voltage system. In this case, an arbitrary power supply voltage can be generated by changing the duty, as a mean value of rectangular pulse-wave voltage, or the direct current component of the rectangular pulse-wave, becomes the power supply voltage of the low voltage system.
In addition, as a load to which it is difficult to apply the high voltage for example, an ignition coil is cited. This ignition coil should be supplied with a low voltage, but a function error may occur when the aforementioned rectangular pulse-wave voltage is supplied. Therefore, in the conventional power supply for a vehicle, the rectangular pulse-wave voltage to be output from the voltage converter is smoothed by a capacitor before supplying to the ignition coil.
However, when the rectangular pulse-wave voltage from the voltage converter is smoothed by a smoothing capacitor, a ripple generation can not be inhibited, and a capacitor of large capacity would be necessary to suppress this ripple sufficiently. As a result, there is a drawback that the power supply for a vehicle become expensive and increase in size.
Moreover, in the aforementioned conventional power supply for a vehicle, a ripple generates also in the voltage at the battery, as current from the battery is on or off by the switching element. In order to remove this ripple, in this conventional power supply for a vehicle, such a control is performed that the current flowing out from the battery is always constant, by operating the plurality of loads successively. However, there is a drawback that, when a number of loads are driven simultaneously, a power necessary for each load can not be obtained, as, the plurality of loads are operated successively and the duty of respective voltage pulse-wave is limited.
It can also be devised to use a conventional battery as smoothing capacitor for absorbing the ripple at the battery, however, this battery can not absorb ripple sufficiently, with full charge and without load. Therefore, in this case also, a smoothing capacitor with large capacity will be required.
Moreover, the voltage converter, consuming power for its operation, lowers the conversion efficiency when the load current is low.
The present invention has been made to solve these problems, and it is an object of the present invention to provide a power supply for a vehicle whose low power in its power consumption and small sizing are allowed and which is inexpensive.
An aspect of the invention provides the following power supply apparatus for a vehicle. The apparatus includes a first battery for supply of power at a first voltage. The apparatus includes a second battery for supply of power at a second voltage to a load. The apparatus includes a converter between the first battery and the second battery. The converter is for conversion of power between the first voltage and the second voltage in magnitude. The apparatus includes a controller for operation of the converter in dependent on a first current in magnitude through the load and a second current in magnitude at an output terminal of the second battery.
Preferably, the power supply apparatus further includes: a first sensor for detection of the first current in magnitude; and a second sensor for detection of the second current: in magnitude. The controller stops the converter when a first detected current is less in magnitude than first specified and a second detected current is less in magnitude than second specified.
Preferably, the power supply apparatus further includes: a switch between the second battery and the load for operation in response to the controller; and a second sensor for detection of the second current in magnitude. The controller stops the converter when the switch is opened and a second detected current is less in magnitude than specified.
Preferably, the power supply apparatus further includes a third sensor for detection of a third current at an output terminal of the converter. The controller stops the converter when a third detected current is less in magnitude than specified and has a ripple greater in level than specified.
Preferably, the converter is started when power is started to be supplied to the load.
Preferably, the controller operates the converter to convert power from the second voltage to the first voltage in dependent on the first current and the second current in magnitude for charge of the first battery.
Preferably, the power supply apparatus further includes a generator for generation of power and for charge of the first battery. The controller operates the generator to be reduced in amount of power generation.
Preferably, the power supply apparatus further includes another load for supply of power from the first battery.
As described above, according to the aspect of the invention, the power supply apparatus for a vehicle is to be reduced Ln its power consumption, as the voltage converter is stopped in the voltage conversion operation according to the first and second currents in magnitude flowing at the output terminal of the second battery and through the second load, and the voltage converter does not consume power. In addition, no ripple is generated in the output with a second voltage from the second battery, when the voltage converter is stopped in voltage conversion operation, and the second battery is to be used as a smoothing capacitor during the voltage conversion operation of the voltage converter, making unnecessary to dispose another smoothing capacitor separately, in any state. As a result, the power supply apparatus for a vehicle is to be reduced in size, and price.
According to the preferable aspect of the invention, the voltage converter is stopped in the voltage conversion operation when the first current in magnitude to be detected by the first sensor is equal or inferior to a first predetermined magnitude (no load state) and the second current in magnitude to be detected by the second sensor is equal or inferior to a second predetermined magnitude (full charged load state), allowing to deploy the identical function and effect as the aforementioned invention of the first aspect.
In addition, according to the preferable aspect of the invention, it is unnecessary to detect the first current in magnitude flowing through the load, as the controller indicates the switch to open, allowing to detect the second battery in no load state. As a result, the first sensor becomes unnecessary, and the power supply apparatus for a vehicle is constituted at a low cost.
According to the preferable aspect of the invention, the voltage converter is stopped in its voltage conversion operation when the third current in magnitude to be detected by the third sensor is equal or inferior to a predetermined magnitude and has ripple equal or superior to a predetermined level, making unnecessary to detect the second current in magnitude flowing at the output terminal of the second battery. As a result, the second sensor for detecting the second current in magnitude flowing at the output terminal of the second battery is unnecessary, allowing to compose the power supply for a vehicle at a low cost.
According to the preferable aspect of the invention, the second battery is to be prevented from its excessive charge at no load thereof, as the voltage converter starts the voltage conversion operation when power supply to the load starts from a stop state thereof. The second battery is prevented from voltage drop to occur when the power supply starts.
In addition, according to the preferable aspect of the invention, the voltage converter starts the voltage conversion operation from the second voltage to the first voltage in magnitude according to the first and second currents in magnitude flowing at the output terminal of second battery and through the load, and the first battery starts to be charged. Therefore, for instance, the first battery is charged when the second battery decreases in the second current flowing at its output terminal to be in the full charge state, and at the identical time, the load decreases in the first current flowing therethrough to be in no load state, allowing to keep the first battery in charge state all the times. Moreover, the second battery is prevented from deterioration due to additional take-out of current, when the second battery in its remaining capacity is low.
According to the preferable aspect of the invention, the first battery is to be prevented from an excessive discharge, as the voltage converter starts the inverse voltage conversion operation according to the first and second currents in magnitude flowing at the output terminal of the second battery and through the load, and the first battery starts to be charged. In addition, the inverse voltage conversion operation is performed when the second battery is in no load state and full charge state, and the second battery is prevented from deterioration due to additional take-out of current when the second battery in its remaining capacity is low.
According to the preferable aspect of the invention, the controller controls the first battery to be charged when the second battery is in no load state and full charge state, and further, the controller controls the generator to be reduced in its power consumption. Thus, the generator is to be reduced in its amount of power generation and is to be improved in its fuel cost.